The Cu—Ga sputtering target has been the essential material for producing a solar cell with the Cu—In—Ga—Se quaternary alloy film (CIGS film) as its light-absorbing layer by so-called “selenization method.” The selenization method is a method of forming a film of CuInGaSe compound, for example, by heating a laminated film, in which CuGa is sputtered at about 500 nm and an In film is sputtered on the sputtered CuGa at the thickness of about 500 nm, in H2Se gas at 500° C. to diffuse Se in CuGaIn (for example, refer Patent Literature 1 (PTL 1)).
Conventionally, in the CuGa sputtering target with high density and high Ga content used for formation of the light-absorbing layer, particularly in the case where Ga content exceeds 28 atomic %, the highly-densified sputtering target becomes very hard and inductile since the deposition ratio of the brittle γ phase with poor workability is increased. Particularly, in the case where it is a casting by melting and casting, cutting work is difficult since cracking or fracturing occur during surface machining by cutting. Thus, machining has to be done by grinding work. Because of this, machining speed of the target is slow, and machining of the target with a complex shape is very difficult.
Under the circumstances described above, a sputtering target with a high-Ga-containing Cu—Ga binary alloy having a two-phase coexistence microstructure, in which a high-Ga-containing Cu—Ga binary alloy grain (high-Ga phase) is surrounded by a grain boundary phase made of a low-Ga containing Cu—Ga binary alloy (low-Ga phase), is proposed (for example, refer Patent Literature 2 (PTL 2)). The sputtering target has a composition containing 30 to 60 mass % of Ga and a balance made of Cu. The high-Ga-containing Cu—Ga binary alloy grain contains 30 mass % or more of Ga and a balance made of Cu. The low-Ga containing Cu—Ga binary alloy contains 15 mass % or less of Ga.
In the above-explained high-Ga-containing Cu—Ga binary alloy sputtering target, cracking or fracturing during cutting are prevented by having the above-explained two-phase coexistence microstructure in which the brittle γ phase is surrounded by the low-Ga phase with excellent malleability. As a result, good yield is obtained.
On the other hand, in order to improve efficiency of electric power generation of the light-absorbing layer made of Cu—In—Ga—Se quaternary alloy film, sodium (Na) addition to the light-absorbing layer is proposed (for example, refer PTL 2 and Non Patent Literature 1 (NPL 1)). In this proposal, it is disclosed that the Na content in the precursor film (Cu—In—Ga—Se quaternary alloy film) is generally set to about 0.1%.